(1) Field of the Invention
The present invention relates to telephone subscriber line apparatus and more particularly to a circuit for determining the on-hook and off-hook status of a telephone subscriber loop.
(2) Description of the Prior Art
In order to provide connection of the switching network to a subscriber station, the central office must be able to detect when the subscriber lifts his handset to the off-hook position to initiate a call, or to answer a call when the central office rings his phone. Once the off-hook condition has been established the central office must also be able to detect the on-hook condition whether caused by placement of the handset in the on-hook position or a series of on-hook and off-hook dial pulses caused by operation of the dial or other calling device.
This problem has traditionally been solved by insertion of a relay in series in the loop which operates when the subscriber completes the loop by lifting his handset. A contemporary approach has been to use solid state devices such as optically-coupled transistors as disclosed in U.S. Pat. No. 3,829,619 issued Aug. 13, 1974, to S. W. Close, et al. Another recent solution has been to use operational amplifiers as disclosed in U.S. Pat. No. 3,941,939 issued Mar. 7, 1976, to P. L. Holmes, et al and U.S. Pat. No. 3,914,556 issued Oct. 21, 1975, to F. W. Frazee.
With further advances in technology, Hall effect devices are now employed in circuits for detection of loop current as disclosed in U.S. Pat. No. 4,020,294 issued Apr. 26, 1977, to R. Kitajewski, et al. In U.S. Pat. No. 4,022,980 also issued to R. Kitajewski, et al, on May 10, 1977, a Hall effect device was disclosed in a ring trip detection circuit.
These solutions to the loop dial pulse and ring trip detection problem have traditionally been accomplished through the use of two distinct circuits. Separation was required between the loop/dial pulse circuit and the ring trip circuit because of the differences in sensitivities required for determining loop status and ring trip.
A combination ring trip and dial pulse detection circuit was disclosed in U.S. Pat. No. 3,838,223 issued Sept. 24, 1974, to D. Q. Lee, et al. This circuit required the use of an optical-coupler, a plurality of logic gates, a delay circuit and a pole change circuit.
The applicants co-pending U.S. patent application, "Loop, Dial Pulse and Ring Trip Detection Circuit", , Ser. No. 865,761, filed on Dec. 29, 1977 discloses a novel use of a combined Hall effect device and adjustable resistance shunt. However, the shunt arrangement shown required adjustment for varying loads and limited the total ringer load that could be connected to the subscribers telephone line.
Accordingly, it is the object of this invention to provide a circuit which can detect both loop/dial pulse and ring trip conditions and use a minimum member of components, to operate without the need for separation of the two detection functions by overcoming the problem of different sensitivities for loop status detection and ring trip detection, to operate without requiring adjustment for different load conditions and operable over a wide spectrum of different load conditions.